1. Field of the Invention
This invention relates to an electronic still-picture camera having a frame-record mode, a field-record mode, and a successive (continuous) photographing operation.
2. Description of the Prior Art
A conventional electronic still-picture camera has a frame-record mode in which image signals for two interlaced fields using two concentric tracks are recorded, and a field-record mode in which image signals for one field using only one track are recorded.
In such an electronic still-picture camera, there is a problem that, when the image mode is set in the frame mode, the time for removing unnecessary charges, the exposure time, the recording time and the like are longer than those in the field mode. Therefore, high-speed successive photography can not be performed. That is, while in the frame recording mode, if unnecessary charges of all picture elements for a field by field sequential reading field by field are simultaneously removed, a difference in dark current is produced since read timing of an odd-numbered field is shifted by one field from that of an even-numbered field. In order to reduce this difference, it is necessary to shift the timing for removing unnecessary charges by an amount corresponding to a difference in read time (time for one field), and the time for clearing becomes longer by one field period. Furthermore, reading is performed by adding signals of two lines in the field mode, while read is performed in a unit of one line in the frame mode. Accordingly, the sensitivity of an imaging device in the frame mode becomes 1/2 of that in the field mode. Hence, exposure time in the frame mode becomes twice that in the field mode. In the frame mode, since crosstalk is generated if it is intended to simultaneously record image signals for two fields on two tracks, sequential recording is desirable. If sequential recording is performed, however, recording time becomes twice that in the field mode. Moreover, in the frame mode, if it is intended to perform sequential recording by shifting heads, there is a need for delay means for delaying the succeeding field signal for the period of time required for the head to shift. Hence, it is desirable to unify two heads. In that case, however, the head shift must be performed by two tracks, and there is the problem that extra time is also required.
The camera is configured such that a field picture is recorded on an SV (still-video) floppy disk (rotating at a constant angular velocity) in synchronization with the rotating phase of the SV floppy disk. Hence, it is not simply better to shorten the time necessary for processes other than recording, because frame speed for successive photographs is not improved unless the time is shortened by 1 V unit (a time for performing one-field recording, that is, a time for one rotation of an SV floppy disk). In the NTSC system (a system of 60 fields/second), for example, since it is realistically impossible to make the necessary time for processes other than recording (for example, track feeding of heads) zero, the maximum realistic frame speed for successive photographs becomes 30 frames/second (1 V for recording, and 1 V for other processes) in the field mode, and 20 frames/second (2 V for recording, and 1 V for other processes) in the frame mode. If it takes more than 1 V for processes other than recording, it becomes impossible to maintain the above-described frame speed (even if the time is 17 msec) (20 frames/second if 1 V for recording and 2 V for other processes in the field mode, and 15 frames/second if 2 V for recording and 2 V for other processes in the frame mode).
Considering the actual state of head-feeding mechanisms, the necessary time for charging a mechanical shutter, and the running time of the screen during exposure, there are required 20 msec-40 msec for moving the head one track, 20 msec-50 msec for charging the mechanical shutter, and 5-15 msec for running the screen during exposure. It is not so difficult to shorten this time itself, but such high speed results in a higher electric-power consumption since all of the above-described processes are accompanied by mechanical movements. Hence, in a portable electronic camera, if these movements are always performed at high speed for the sake of frame speed during high-speed successive photographing operations, the number of usable frames per one set of batteries is usually reduced. Changing the moving speed of these mechanical moving parts during successive photographing operations and during one-frame photographing results in a complex mechanical structure, high cost, and a decrease in reliability.
If the frame speed during successive photographing operations is estimated by these values, it is only possible to photograph 1 V+45 msec-105 msec.fwdarw.4 V-8 V (15 frames/second-7.5 frames/second) for the field mode, and 2 V+65 msec-145 msec.fwdarw.6 V-11 V (10 frames/second-5.5 frames/second) for the frame mode, if all operations are performed time serially. Even if simultaneous operations are executed among these processes (shutter charge, exposure and track feeding), and neglecting an increase in instantaneous current consumption (an increase in current consumption decreases the voltage which can be effectively utilized due to the existence of an internal resistance of a battery, and this fact also results in a decrease in the number of usable frames), it is only possible to photograph 1 V+25 msec-65 msec.fwdarw.3 V-5 V (20 frames/second-12 frames/second) for the field mode, and 2 V+40 msec-80 msec.fwdarw.5 V-7 V (12 frames/second-8.5 frames/second) for the frame mode. These values are fairly slow compared with the maximum desirable frame speeds (calculated values based on realistic assumptions) of 30 frames/second in the field mode, and 20 frames/second in the frame mode.
As described above, it is impossible to perform a high-speed successive photographing operation when the frame mode is set. If photographing has been started without noticing this situation, a shutter opportunity may be missed.